The wall of the severely dilated bronchus, which can be identified by the partially circumferential residual epithelium,  shows marked thinning with loss of smooth muscle and cartilage.

The bronchial wall shows acute and chronic inflammation. The lumen is filled with inflammatory exudate, primarily acute.  There is a long list of causes of bronchiectasis including inborn errors of metabolism, infections, obstruction, and immune reactions.

Patients with bronchiectasis have permanent dilation of bronchi and bronchioles secondary to destruction of mural smooth muscle and elastic tissue. There is also loss of cartilage in larger bronchi. These changes are the sequelae of chronic necrotizing infection. Congenital diseases associated with bronchiectasis include cystic fibrosis, Kartegener syndrome, intralobular sequestration, and immunodeficiency states. Other conditions leading to bronchiectasis include bronchial obstruction (tumors or foreign bodies) and bacterial or viral or fungal organisms causing necrotizing pneumonia, Cystic fibrosis is the most common antecedent of bronchiectasis in children and young adults. Patients with bronchiectasis secondary to congenital diseases usually present in childhood with a cough productive of much mucopurulent sputum which may be blood-streaked. Usually there is a history of recurrent lower respiratory tract infections. In such cases there are likely to be diffuse changes involving both lungs, especially in the lower lobes. With chronicity the patients will note weight loss. Among bacterial organisms associated with bronchiectasis are Klebsiella organisms, S. aureus, Mycoplasma pneumoniae, Mycobacterium tuberculosis, and other mycobacterium species. After bronchiectasis has developed, subsequent infections may be due to the same organisms but Haemophilus species and Pseudomonas species become common offending agents. High-resolution CT scanning is presently the best imaging technique for the diagnosis of bronchiectasis.